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In a book i have there's an exercise that want to find the output of a system for a given signal

system: $ 2y(n)=2x(n)+x(n+1)+x(n-1) $

input : $ Χ=[\underline1, -1, 2, 0, -2, 0, 1 ]$

$Χ1=Χ[0] $

What represent's the underlined value?

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    $\begingroup$ Just out of curiosity: which book are you referring to? (because I've never seen this notation) $\endgroup$ – Matt L. Jun 18 '14 at 8:46
  • $\begingroup$ it's an internal book from my uni. $\endgroup$ – Giannis Foulidis Jun 18 '14 at 20:00
  • $\begingroup$ Very interesting concept. $\endgroup$ – jojek Jun 28 '14 at 18:52
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An arrow or an underline is used to indicate the value at $n=0$. Example,

$$y(n) = \begin{array}{ccccc}[&9 &8 &\underline7 &6 &5 &]\end{array}$$ means that, $$\begin{array}{ccccc}&y(-2) = 9, &y(-1) =8, &y(0)=7, &y(1) = 6 &\mathrm{and} &y(2) =5\end{array}$$

In your case, it says that $$\begin{array}{ccccc}&X(0) = 1, &X(1) =-1, &X(2)=2, &\ldots\end{array}$$

Note: No underline or arrow is equivalent to underlining the first value.

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